FIELD OF THE INVENTION
The present invention relates to a noise-canceling method for canceling the engine noise at a predetermined position (observation point) in an automotive vehicle and, more particularly, to a noise-canceling method for canceling harmonic components of a level higher than 2 among harmonic components of an engine rotational speed, which are contained in the engine noise, so as to provide a comfortable environment in the passenger compartment of the automotive vehicle.
A known method of dealing with noise involves using a sound-absorbing material (this is a method of passive control). With a method that relies upon use of a sound-absorbing material, however, forming a silent area of little noise is troublesome and low-pitched sounds are not eliminated effectively. In particular, when noise within the passenger compartment of an automotive vehicle is prevented by passive control, the vehicle increases in weight and the noise cannot be eliminated effectively.
For this reason, active-control methods in which a noise-canceling sound, whose phase is the opposite of the noise, is emitted from a speaker so as to reduce the noise have become the focus of attention. Additionally, these methods are being put into practical use in factory and office interiors. Systems for reducing noise by active control have been proposed for the passenger compartments of automotive vehicles as well.
FIG. 12 is a block diagram of an apparatus for achieving the cancellation of sound. As shown in FIG. 12, an engine 11 which is a source of noise has its rotational speed R sensed by an rotational speed sensor 12. The output R of the sensor 12 is applied to a reference signal generator 13, which generates a sinusoidal signal having a fixed amplitude and a frequency that conforms to the rotational speed R of the engine 11. The sinusoidal signal serves as a reference signal x.sub.n. When an engine is a source of noise, the noise generated by rotation of the engine has periodicity (is periodic noise) and the frequency of the noise is dependent upon the engine rotational speed. In the case of a four-cylinder engine, for example, the frequency of periodic noise generated within the passenger compartment is 20 Hz when the rotational speed is 600 rpm (=10 rps) and 200 Hz when the rotational speed is 6000 rpm (=100 rps). These are second harmonics of the engine speed. Accordingly, the reference signal generator 13 stores the sinusoidal data in a ROM and generates the reference signal x.sub.n by reading out and delivering this data as necessary. The timing at which this data is read out and delivered is controlled in accordance with the engine rotational speed R so that the reference signal output will have a frequency conforming to the engine rotational speed R.
The reference signal x.sub.n generated by the reference-signal generator 13 is applied to a noise-canceling controller 14 as an input. Also fed into the controller 14 is an error signal e.sub.n, which is a composite-sound signal that is a synthesis of noise S.sub.n and a noise-canceling sound S.sub.c at a noise-canceling position (an observation point, such as a point in the vicinity of the ears of the driver) within the passenger compartment. The noise-canceling controller 14 outputs a noise-canceling signal N.sub.c by executing an adaptive signal processing so as to minimize the error signal e.sub.n. The controller 14 includes an adaptive signal processor 14a; an adaptive filter 14b constructed as a digital filter; a DA converter 14c for converting the output of the adaptive filter 14b (noise-canceling signal N.sub.c) into an analog noise-canceling signal; and a filtered-X signal producing filter 14d for producing a filtered-X signal (reference signal for a signal processing) r.sub.n by superimposing, on the reference signal x.sub.n, the propagation characteristic (transfer function) of a secondary-sound propagation system extending from a speaker to the noise-canceling point. The reference numeral 15 denotes a power amplifier for amplifying the noise-canceling signal, numeral 16 a canceling speaker for emitting the noise-canceling sound S.sub.c ; numeral 17, an error microphone which is disposed at the noise-canceling point so as to detect the composite sound of the noise S.sub.n and the noise-canceling sound S.sub.c, and output a composite-sound signal as the error signal e.sub.n ; numeral 18, a secondary-sound propagation system (noise-canceling sound propagation system); numeral 19, an amplifier for amplifying the error signal e.sub.n ; numeral 20, a low-pass filter for eliminating noise signals outside the band of periodic noise; and numeral 20', an AD (Analog to Digital) converter for converting the output of the low-pass filter 20 into a digital signal.
The error signal e.sub.n at the noise-canceling point and the filtered-X signal r.sub.n, which is output from the filter 14d enter the adaptive signal processor 14. The adaptive signal processor 14a decides the coefficients of the adaptive filter 14b by using these two signals to execute an adaptive signal processing in such a manner that the noise at the noise-canceling point is canceled out. For example, the adaptive signal processor 14a decides the coefficients of the adaptive filter 14b in accordance with a well-known filtered-X LMS (least mean square) algorithm so as to minimize the error signal e.sub.n that has entered from the error microphone 17. In accordance with the coefficients decided by the adaptive signal processor 14a, the adaptive filter 14b subjects the reference signal x.sub.n to a digital filtering processing so that the DA (Digital to Analog) converter 14c will deliver the canceling-sound signal N. It should be noted that the reference signal x.sub.n must be a signal having a high correlation with respect to the noise S.sub.c to be canceled; sounds having no correlation with the reference signal are not canceled out.
When the engine 11 rotates, its rotational speed R is detected by the rpm sensor 12, the reference signal generator 13 generates the reference signal x.sub.n [see (a) in FIG. 13], whose frequency conforms to the engine rotational speed R, and the reference signal x.sub.n enters the noise-canceling controller 14.
At this time the periodic engine sound (periodic noise) generated by the engine 11 reaches the noise-canceling point upon propagating through space having a noise propagation system (a primary-noise propagation system) that exhibits a predetermined transfer function. Accordingly, the noise (engine sound) S.sub.n at the noise-canceling point has a slightly lower level and a slight delay, as illustrated at (b) in FIG. 13.
Initially, the noise-canceling controller 14 produces the noise-canceling signal N.sub.c so as to have a phase opposite that of the reference signal x.sub.n as a result of which the canceling speaker 16 outputs the noise-canceling sound S.sub.c shown at (c) in FIG. 13, by way of example. However, since the level and phase of the noise S.sub.n are displaced somewhat from the level and phase of the noise-canceling sound S.sub.c, the noise is not canceled out by the noise-canceling sound S.sub.c and, hence, the error signal e.sub.n is generated. The noise-canceling controller 14 determines the coefficients of the adaptive filter 14b by performing an adaptive signal processing in such a manner that the error signal e.sub.n is minimized. In an ideal case, the phase of the noise-canceling sound S.sub.c will be opposite that of the noise S.sub.n and the levels thereof will be in agreement, as shown at (d) in FIG. 13, so that the noise is canceled out.
In order to simplify the description, the foregoing example deals with one noise source, one source (the speaker) for generating the noise-canceling sound, and one noise-canceling point (the observation point). In actuality, however, there is more than one noise source and more than one point (observation point) at which noise is desired to be canceled. In this case, more than one speaker is necessary since noise at a plurality of points cannot be canceled with only one speaker. FIG. 14 is a block diagram of a conventional noise-canceling apparatus for a case in which there are K-number of noise sources, M-number of speakers and L-number of observation points.
The reference numeral 21 denotes a noise-canceling controller having a DSP (digital signal processor) structure (which corresponds to the noise-canceling controller 14 in FIG. 12) that operates so as to cancel out noise at each of a number of observation points. The reference numeral 22 denotes a primary-sound hypothetical propagation system (noise propagation system), which expresses systems along which noise is propagated from each noise source (not shown) to each observation point. The reference numeral 23 represents a secondary-sound propagation system (noise-canceling sound propagation system), which expresses systems along which the noise-canceling sound is propagated from each speaker to each observation point. The system 23 includes the characteristics of the speakers (not shown). The reference numeral 24 designates a signal synthesizer, which implements the function of a microphone at each observation point. The signal synthesizer 24 includes adders 24.sub.1 -24.sub.1 ' corresponding to a microphone at a first observation point, adders 24.sub.2 -24.sub.2, corresponding to a microphone at a second observation point, . . ., and adders 24.sub.L -24.sub.L ' corresponding to a microphone at an L-th observation point. Further, d.sub.d1n -d.sub.dLn represent external noise that is not the object of cancellation at each of the observation points.
The noise-canceling controller 21 includes a multiple-input/multiple-output adaptive filter (hereinafter referred to simply as an adaptive filter) 21a for inputting noise-canceling signals Y.sub.a1n -Y.sub.aMn to the speakers upon being provided with inputs of reference signals x.sub.a1n -x.sub.aKn (output by a reference signal generator, not shown) conforming to the noise components generated by the noise sources; a filtered-X signal producing filter 21b, which is fabricated using the elements (propagation elements) of a transfer-function matrix of the secondary-sound propagation system 23, this filter being provided with inputs of the reference signals x.sub.a1n -x.sub.aKn conforming to the noise generated by the noise sources; and an adaptive signal processor 21c, which is provided with inputs of error signals e.sub.1n -e.sub.Ln prevailing at the observation points and filtered-X signals r.sub.111n -r.sub.LMKn output by the filter 21b, for deciding the coefficients of the adaptive filter 21a by executing an adaptive signal processing using these input signals so as to cancel out the noise at each observation point.
FIG. 15 is a diagram for describing the primary-sound hypothetical propagation system 22. As shown in FIG. 15A, the noise generated by K-number of noise sources NG.sub.1 -NG.sub.K reaches microphones (MIC.sub.1 -MIC.sub.L), which are provided at the respective observation points, upon propagating through the primary-sound propagation system 22 having a predetermined frequency and predetermined phase characteristics. Accordingly, if we let H.sub.ji represent the transfer characteristic of a propagation system in which noise from an i-th noise source NG.sub.i reaches a j-th microphone MIC.sub.j, the primary-noise hypothetical propagation system 22 will be expressed as shown in FIG. 15B and the transfer-function matrix (H) thereof will be as follows: ##EQU1##
Each element H.sub.ij of the transfer-function matrix (H) is implemented by a FIR-type digital filter shown in FIG. 16. More specifically, each element is realized by a digital filter comprising delay elements DL for successively delaying the input signal by one sampling period, multipliers ML for multiplying the outputs of the delay elements by coefficients h.sub.0, h.sub.1, h.sub.2, . . . , and adders AD for adding the outputs of the multipliers.
FIGS. 17A and 17B are views for describing the secondary-noise propagation system 23. As shown in FIG. 17A, noise-canceling sounds generated by speakers SP.sub.1 -SP.sub.M arrive at the microphones MIC.sub.1 -MIC.sub.L, which are provided at the respective observation points, upon propagating through the secondary propagation system 23 having a prescribed frequency and predetermined phase characteristics. Accordingly, if we let C.sub.ji represent the transfer characteristic of a secondary-noise propagation system in which a canceling sound based upon an i-th noise-canceling signal Y.sub.ain reaches the j-th microphone MIC.sub.j, the secondary-noise propagation system 23 will have the form of the model shown in FIG. 17B and the transfer-function matrix (C) thereof will be as follows: ##EQU2##
Each element of the transfer-function matrix (C) is implemented by a FIR-type digital filter shown in FIG. 16, just as in the case of the primary-sound hypothetical propagation system 22. More specifically, each element is realized by a digital filter comprising delay elements DL for successively delaying the input signal by one sampling period, multipliers ML for multiplying the outputs of the delay elements by coefficients c.sub.0, c.sub.1, c.sub.2, . . . , and adders AD for adding the outputs of the multipliers.
FIG. 18 is a block diagram showing the filtered-X signal-producing filter 21b fabricated using each element C.sub.ij of the transfer-function matrix (C) of the secondary-sound propagation system 23.
The adaptive signal processor 21c updates the coefficients of the adaptive filter 21a by executing an adaptive signal processing based upon the reference signals x.sub.a1n -x.sub.aKn and the signals e.sub.n -e.sub.Ln that are a composite of the noise and the noise-canceling sounds at each of the observation points, and the adaptive filter 21a, to which the reference signals x.sub.a1n -x.sub.aKn are applied as inputs; generates the noise-canceling signals Y.sub.a1n -Y.sub.aMn and applies these signals to the speakers to cancel out the sound at each observation point.
The noise-canceling signals Y.sub.a1n -Y.sub.aMn output by the adaptive filter 21a do not reach the observation points as is. Rather, they reach the observation points upon being influenced by the frequency and phase characteristics of the secondary-sound propagation system 23. As a consequence, the adaptive signal processor 21c performs highly sophisticated noise-canceling control, not by using the reference signals x.sub.a1n -xaK.sub.n as is, but by employing a filtered-X LMS (multiple-error filtered X LMS, referred to as an "MEFX LMS") algorithm, which uses signals obtained by impressing the characteristics of the secondary-sound propagation system 23 on the reference signals. In other words, on the basis of the filtered-X LMS algorithm, the adaptive signal processor 21c updates the coefficients of the adaptive filter 21a using signals r.sub.111n -r.sub.LMKn, which are a result of filtering the reference signals x.sub.a1n -xaK.sub.n by the filter 21b, and the composite-sound signals (error signals) e.sub.1n -e.sub.Ln at the observation points.
In FIG. 18, C.sub.ij represents a FIR-type digital filter for realizing each element C.sub.ij (see FIG. 17) of the transfer-function matrix (C) in the secondary-sound propagation system 23. The filter 21b is adapted so as to output the filtered-X signals r.sub.111n -r.sub.LMKn upon impressence all of the propagation elements upon each of the refersignals signals x.sub.a1n -x.sub.aKn (i.e., passing each reference signals through filters corresponding to all of the propagation elements). More specifically, the propagation elements C.sub.11 -C.sub.L1 from the first speaker to all of the observation points are made to act upon the reference signal x.sub.a1n to produce the filtered-X signals r.sub.111n -r.sub.L11n ; the propagation elements C.sub.12 -C.sub.L2 from the second speaker to all of the observation points are made to act upon the reference signal xa1n to produce the filtered-X signals r.sub.121n -r.sub.L21n, . . . ; and the propagation elements C.sub.1M -C.sub.LM from the M-th speaker to all of the observation points are made to act upon the reference signal xa1n to produce the filtered-X signals r.sub.1M1n -r.sub.LM1n. All of the propagation elements are made to act upon each of the reference signals x.sub.a2n, x.sub.a3n, . . . x.sub.aKn in a similar manner. This may be expressed as follows: ##EQU3##
FIG. 19 is a block diagram showing the multiple-input/multiple-output adaptive filter 21a, which has a structure similar to that of the primary-sound hypothetical propagation system 22 or secondary-sound propagation system 23. FIR-type digital filters are shown at A11.sub.n -A.sub.MKn. By way of example, each of these filters may be realized by delay elements D.sub.L1, D.sub.L2. . . for successively delaying the input signal by one sampling period, multipliers ML1, ML2, ML3 . . . for multiplying each delay-element output by coefficients a.sub.0, a.sub.1, a.sub.2. . ., and adders AD.sub.1, AD.sub.2. . . for adding the multiplier outputs. The number of delay stages is not limited to two.
The noise-canceling signal y.sub.a1n input to the first speaker is obtained by inputting the reference signals x.sub.a1n -x.sub.aKn to the digital filters A.sub.11n -A.sub.1Kn and then adding. The noise-canceling signal y.sub.a2n input to the second speaker is obtained by inputting the reference signals x.sub.a1n -x.sub.aKn to the digital filters A.sub.21n -A.sub.2Kn and then adding. The noise-canceling signal y.sub.aMn input to the M-th speaker is obtained by inputting the reference signals x.sub.a1n -x.sub.aKn to the digital filters A.sub.M1n -A.sub.MKn and then adding.
When each of the FIR-type digital filters A.sub.11n -A.sub.MKn in the adaptive filter 21a is composed of three coefficients (two delay stages), the adaptive signal processor 21c decides the values of the coefficients by executing an adaptive signal processing for each of the three coefficients of the FIR-type digital filters A.sub.11n -A.sub.MKn. That is, the adaptive signal processor decides coefficients a.sub.0, a.sub.1, a.sub.2 by performing the following operation with regard to these coefficients a.sub.0, a.sub.1, a.sub.2 of one FIR-type digital filter A.sub.ijn : ##EQU4##
In the equation (1), the symbol (n) signifies the value at the present sampling time, (n-1) the value at the preceding sampling time, (n-2) the value at the sampling time two samplings before the present sampling time, and (n+1) the value from the present time to the next sampling time. Accordingly, the symbol R.sub.ij (n-2) signifies the output of the filter 21b that conforms to the reference signal at the sampling time two samplings before the present sampling time, R.sub.ij (n-1) signifies the output of the filter that conforms to the reference signal at the preceding sampling time, and R.sub.ij (n) signifies the output of the filter that conforms to the reference signal at the present time. The symbol .mu. represents a constant (step-size parameter) of not more than 1 for deciding a step (degree) at which the coefficient of the adaptive filter is updated, and .mu. is set to an appropriate value in accordance with the noise,canceling system. The larger the value of the step-size parameter .mu. is, the faster the coefficient of the adaptive filter approaches the optimum value and the better the followability becomes. However, an overshoot is generated when the value of .mu. approaches the optimum value, which deteriorates the stability. On the other hand, the smaller the value of the step-size parameter .mu. is, the slower the coefficient of the adaptive filter approaches the optimum value and the worse the followability becomes. However, the overshoot is small when the value of .mu. approaches the optimum value, thereby ameliorating the stability. The symbol e.sub.n represents the signal (error signal) that is the composite of the noise and the noise-canceling sound at each of the L-number of observation points.
In accordance with this noise-canceling apparatus, the adaptive signal processor 21c decides the coefficients of the FIR-type digital filters A.sub.11n -A.sub.MKn, which constitute the adaptive filter 21a, by executing an adaptive signal processing based upon the filtered-X signals r.sub.111n -r.sub.LMKn, which are output by the filter 21b and the composite-sound signals (error signals) e.sub.1n -eL.sub.n that are composites of the noise and noise-canceling sounds at the respective observation points. The adaptive filter 21a, to which the reference signals x.sub.a1n -x.sub.aKn are applied, generates the noise-canceling signals Y.sub.a1n -Y.sub.aMn and applies these signals to the speakers SP.sub.1 -SP.sub.M (FIG. 17). Each speaker generates a noise-canceling sound to cancel out the noise at each observation point.
FIG. 20 is a block diagram illustrating the details of the conventional noise-canceling apparatus for a case in which there is one noise source (K=1), two speakers (M=2) and two observation points, i.e., two microphones (L=2). This conventional noise-canceling apparatus is used, for example, for canceling the engine noise at the two front seats (driver's seat and passenger's seat) in an automotive vehicle.
The reference numeral 21a denotes the adaptive filter, which is composed of two FIR-type digital filters A.sub.11n, A.sub.21n ; the numeral 21b the filtered-X signal producing filter, which is obtained by using digital filters to construct each of the propagation elements C.sub.11, C.sub.21, C.sub.12, C.sub.22 of the transfer-function matrix of the secondary propagation system; and the numerals 21c-1 and 21c-2, adaptive signal processors (MEFX LMS) for deciding the coefficients of each of the digital filters in the adaptive filter 21a. The symbols SP.sub.1 and SP.sub.2 represent speakers which are placed under the two seats, and MC.sub.1 and MC.sub.2 designate microphones disposed at the observation points (points in the vicinity of the ears of the passengers). The operations of the adaptive signal processor, the adaptive filter, and the filtered-X signal producing filter are executed by one DSP (digital signal processor).
FIG. 21 is a block diagram illustrating the details of the conventional noise-canceling apparatus for a case in which there are one noise source (K=1), four speakers (M=4) and four observation points, i.e., four microphones (L=4). This conventional noise-canceling apparatus is used for canceling the engine noise at the two front seats and the two back seats in an automotive vehicle.
The reference numeral 21a denotes the adaptive filter, which is composed of four FIR-type digital filters A.sub.11n, A.sub.21n, A.sub.12n, A.sub.22n, and numeral 21b denotes the filtered-X signal producing filter, which is obtained by using digital filters to construct each of the propagation elements C.sub.11, C.sub.21, C.sub.31, C.sub.41. . ., C.sub.44 of the transfer-function matrix of the secondary propagation system. Numerals 21.sub.c-1 through 21.sub.c-4 denote adaptive signal processors (MEFX LMS ), SP.sub.1 -SP.sub.4 represent speakers, and MC.sub.1 -MC.sub.4 designate microphones disposed at the observation points. The operations of the adaptive signal processor, the adaptive filter, and the filtered-X signal producing filter are executed by one DSP (digital signal processor).
When canceling the engine noise, only a second harmonic component which is contained in the engine noise is usually canceled. In the case of a four-cylinder engine, however, the engine noise contains not only the second harmonic component but also fourth, sixth, . . . harmonic component, although the levels thereof are lower, as shown in FIG. 22. In addition, harmonic components of odd ordinals are generated in some types of automotive vehicle. The silencing effect produced by a system for which cancels only the second harmonic is therefore insufficient for auditory sensation.
In order to improve the silencing effect, a method of canceling the harmonic components by performing the adaptive signal processing of harmonic components of high levels (for example, second, fourth and sixth harmonic components) among the harmonic components contained in the engine noise, separately from each other may be adopted.
FIG. 23 shows the structure of a conventional noise-canceling apparatus for canceling the second, fourth and sixth harmonic components which are contained in the engine noise. The same reference numerals are provided for the same elements as those shown in FIG. 12. This apparatus is different from that shown in FIG.12 in the following points:
(1) A noise-canceling controller 14-1 for canceling a second harmonic component, a noise-canceling controller 14-2 for canceling a fourth harmonic component, and a noise-canceling controller 14-3 for canceling a sixth harmonic component are provided separately from each other, PA1 (2) Reference that reference signals x.sub.2n, x.sub.4n and x.sub.6n of second, fourth and sixth harmonics, respectively, of the engine rotational speed are generated from the reference signal generator 13 and are input to the noise-canceling controllers 14-1, 14-2, and 14-3, respectively, PA1 (3) that adders 14-4 and 14-5 which add the outputs N.sub.c2, N.sub.c4, and N.sub.c6 of the respective noise-canceling controllers and output the result as a noise-canceling signal N.sub.c are provided, and PA1 (4) that the composite-sound signal (error signal) e.sub.n detected by the error microphone 17 is fed back to each of the noise-canceling controllers 14-1, 14-2 and 14-3. PA1 generating reference signals corresponding to the respective harmonic components of the engine rotational speed which are contained in the engine noise; PA1 generating a noise-canceling signal by executing an adaptive signal processing by using each reference signal and a composite-sound signal and inputting the noise-canceling signal to a speaker; and PA1 outputting a predetermined harmonic component from a speaker for generating a noise-canceling sound or another speaker so that the levels of the harmonic components become approximately equivalent to one another at the noise-canceling point. PA1 executing an adaptive signal processing of the harmonic component of the highest level among the harmonic components of the engine rotational speed which are contained in the engine noise, in accordance with a first coefficient updating equation which does not incorporate an output limiting term for limiting the amplitude of the output signal of the adaptive filter; PA1 executing the adaptive signal processing of other harmonic components in accordance with the a second coefficient updating equation which incorporates an output limiting term for limiting the amplitude of the output signal of the adaptive filter; and PA1 generating a noise-canceling signal by adding the output signals output from adaptive filters. PA1 providing a common noise-canceling controller for the harmonics of all levels; PA1 generating reference signals corresponding to the respective harmonic components of a level higher than 2 among the harmonic components of the engine rotational speed which are contained in the engine noise; PA1 controlling the amplitude of the reference signals so that the lower the level of the harmonic component is, the larger the amplitude of the reference signal corresponding to the harmonic component becomes; PA1 generating a composite reference signal from the reference signals whose amplitudes are controlled; PA1 executing an adaptive signal processing by using the composite-sound signal of the engine noise and the noise-canceling sound at the noise-canceling point, the composite reference signal, and the step-size parameter; and PA1 canceling the engine noise at the noise-canceling point by inputting the noise-canceling signal obtained by the adaptive signal processing to the noise-canceling sound generating source. PA1 providing a first noise-canceling controller corresponding to the harmonic component of the highest level and a second noise-canceling controller corresponding to other harmonic components; PA1 constantly canceling the harmonic component of the highest level by the adaptive signal processing of the first noise-canceling controller; and PA1 canceling the harmonic component decided in accordance with the engine rotational speed or designated by a switch, by the adaptive signal processing of the second noise-canceling controller.
The noise-canceling controllers 14-1, 14-2 and 14-3 for canceling the respective harmonics execute the above-described adaptive signal processings in accordance with the coefficient updating equation (1) on the basis of the reference signals x.sub.2n, x.sub.4n and x.sub.6n, the composite-sound signal e.sub.n and the step-size parameter .mu., decides the coefficients of the respective adaptive filters (not shown), input the reference signals x.sub.2n, x.sub.4n and x.sub.6n to the adaptive filters, and output the noise-canceling signals N.sub.c2, N.sub.c4 and N.sub.c6. The adders 14-4 and 14-5 add the outputs N.sub.c2, N.sub.c4 and N.sub.c6 of the respective noise-canceling controllers and output the result as the noise-canceling signal N.sub.c to the DA converter 14c. The DA converter 14c converts the noise-canceling signal N.sub.c into the analog noise-canceling signal, inputs the analog noise-canceling signal into the speaker, and outputs the noise-canceling sound.
If it is assumed that the fourth harmonic frequency of the current engine rotational speed is represented by f.sub.0, the frequency characteristic of the adaptive filter in the state in which the fourth harmonic is canceled by the fourth harmonic noise-canceling controller 14-2 is as indicated by the solid line in FIG. 24. That is, the frequency characteristic of the adaptive filter of the fourth harmonic noise-canceling controller 14-2 forms a sharp trough at the frequency of f.sub.0. Even if the frequency characteristic of the adaptive filter of the fourth harmonic noise-canceling controller 14-2 forms such a trough, there is no problem if the adaptive control follows a slight change in the engine rotational speed (the fourth harmonic frequency changes from f.sub.0 to f.sub.1) so that the frequency characteristic of the adaptive filter quickly changes from the solid line to the broken line in FIG. 24. If the adaptive control is delayed, however, the gain rapidly increases by g.sub.L and the noise-canceling output for the fourth harmonic is rapidly expanded, so that the engine noise is increased (noise increasing phenomenon) far from being canceled out. The same problem is caused with regard to a sixth harmonic.
After the composition of the outputs of the second, fourth and sixth adaptive filters, the composite-sound signal is converted into an analog signal by the DA converter 14c and input to the speaker. However, the outputs of the fourth and sixth adaptive filters are lower than the output of the second adaptive filter by 10 to 20 dB. Since the DA converter 14c is designed in such a manner that the resolving power thereof is high at the output level of the second adaptive filter, the S/N of the DA converter 14c is low to the fourth and sixth harmonics.
The step-size parameter .mu. in the coefficient updating equation (1) is determined so as to be able to cancel the engine noise while satisfying both the followability and the stability. The second harmonic component which is dominant in the engine noise, that is, which has the highest noise level, is therefore canceled out while satisfying both the followability and the stability. However, when the other harmonic components of a lower level are canceled by using the coefficient updating equation (1), the noise-canceling signals N.sub.c4 .sub.and N.sub.c6 output from the noise-canceling controllers 14-2 and 14-3 sometimes become too large. In such a case, these noise-canceling signals can not cancel the fourth and the sixth harmonic components, and rather produce another noise. For this reason, a method of reducing the values .mu. for the harmonic components other than the second harmonic component in the coefficient updating equation may be adopted. If .mu. is reduced, however, the followability is deteriorated, so that it is impossible to quickly follow and cancel the noise (harmonic components) which changes moment by moment.
In the conventional method of canceling a plurality of harmonic components, it is necessary to provide noise-canceling controllers having the DSP structure for the respective harmonic components, which requires a large-scale hardware structure and raises the manufacturing cost. 0n the other hand, if the adaptive signal processing of each harmonic component is executed by using only one noise-canceling controller, the amount of operation by the DSP becomes very large and it is necessary to reduce the number of taps (number of stages) of the filtered-X signal producing filter or the adaptive filter, which makes it impossible to output a noise-canceling signal that satisfies the followability. Another possible measure is a method of inputting the composite reference signals of all the harmonic components into one noise-canceling controller, thereby executing the adaptive signal processings in block by the noise-canceling controller. In such a method, however, since harmonic components of a high level are mainly canceled, the cancellation of harmonic components of a lower level are insufficient. As a result, the harmonic components of a lower level are offensive to the ear.